As improvements in semiconductor manufacturing technology continue to reduce the size of features formed on substrates in manufacture of integrated circuits and the like, demands on the dimensional metrology used to evaluate the accuracy of manufacturing processes and the like increase accordingly. There is a particular need for test structures and methods for calibrating various types of instruments used for measuring the locations, widths, and spacing of conductors and other structures formed on substrates, features of masks and other tools used to fabricate such structures, and like geometrical characteristics of similar structures.
More particularly, in grandparent application Ser. No. 08/382,973, now U.S. Pat. No. 5,617,340, the problem of measuring the distance between two parallel conductors is addressed in detail. Distances between spaced conductors, and the widths of such conductors, are commonly measured using an imaging instrument, such as an optical microscope or an electron microscope, detecting radiation reflected or scattered by the element to be imaged. In some cases, these instruments may detect radiation transmitted through a radiation-transparent substrate on which the feature is formed.
In many cases of interest, such features are patterned by photolithographic selective removal processes. Where the features are to be conductive, such processes essentially involve the steps of forming a continuous thin layer of semiconductive material, conductive metal, or the like over a substrate, covering the layer with a photoresist mask patterned to define areas of the layer which are to remain, exposing this assembly to an etchant which removes the portions of the layer not protected by the photoresist mask, and then removing the mask, so that only the patterned areas remain of the original continuous layer. Similar techniques are used to form calibration reference grids and like structures used for instrument calibration and other metrologic purposes.
Ideally (from the point of view of accurate metrology as well as that of product manufacture itself), conductors and like features formed on a substrate using such selective removal processes would exhibit regular cross-sectional shapes. For example, the side walls of such ideal conductors might rise at exactly right angles from the substrate to meet the top of the remaining planar upper surface of the layer of conductive material at another right angle. Light would be reflected as if from a mirror from the flat upper surface of the patterned feature to be imaged, and would be reflected in a differing manner from the side walls of the feature, enabling the "corners" of the side wall to be optically detected, and the width, spacing, and like geometrical characteristics of such features accurately measured.
In practice, the side walls of the conductors (and other features) tend to exhibit irregular side wall angles and roughness due to, for example, under-cutting, local variations in the etch rate, and the like. These irregularities in formation of the side walls render the width of the conductor somewhat variable and ill-defined, and similarly complicate evaluation of the width of a feature or the spacing of two features having such irregular side walls using an imaging instrument, for example, an optical microscope, where radiation reflected from or transmitted past the structure is to be detected. Particularly for providing a certifiable reference structure, it would be desirable to provide a method of fabricating a feature on a substrate wherein the side walls of the features were regular and substantially planar, so that the widths, spacing, and other geometrical characteristics of the features thus formed might be unambiguously specified to a high degree of accuracy; such a structure could then be used to calibrate other instruments, such as optical or electron microscopes used in manufacturing processes.
More specifically, there are disclosed in the grandparent application several types of test structures for cross-calibrating imaging instruments, such as electron or optical microscopes, with respect to instruments providing electrical measurements of the same test structure. "Electrical measurements", as referred to in the grandparent application, include measurements made by forcing a current along a conductive member and measuring voltage drops between spaced locations therealong, as well as capacitative, inductive, or impedance measurements of the geometrical characteristics of the conductor.
The parent application Ser. No. 08/409,467 discusses the desirability of a similar cross-calibration capability with respect to scanning tunneling microscopes (STMs), including in the latter term all types of instruments wherein a tunneling current passes between a probe moved over a conductive object being inspected and the object, the tunneling current varying with juxtaposition of the probe to each individual atomic site of the structure being inspected. Such microscopes are becoming increasingly useful for "atomic lattice counting" as a means of measuring the dimensions of a structure, for example. The parent application provides test structures and methods for cross-correlation of an SPM measurement with an electrical measurement, or with an imaging-instrument measurement.
The parent application also recognized the fact that an electrical measurement of "linewidth", i.e., the width of an elongated conductor formed on a substrate, typically provides a value for the average width of the entire conducting line, while a scanning electron microscope (SEM) usually measures linewidth using an algorithm based on calibration using a non-electrically-calibrated grating, and optical and STM methods provide a local "snapshot" of the linewidth at a specific point of measurement. More specifically, optical measurements attempt to provide a measurement of linewidth using two defined points at both edges of an image of the line, followed by analysis of the output signal from an optical detector to calculate the linewidth. Due to the irregularities normally exhibited by the sidewalls of conductors on a substrate, an image of the conductors formed using an optical measurement instrument will exhibit substantial indefiniteness as to the exact location of the edges of the sidewalls of the conductor. Modeling is used to determine where the edges of the line are likely to be. It would be preferable to avoid the uncertainty inherent in the theoretical modelling step.
The art recognizes generally that these various techniques for measuring the same physical parameter give varying results. To a degree, these are explained by the intrinsic characteristics of the different measuring techniques; these differences are also likely due in part to irregularities in the line being measured.
The parent application discloses and claims test structures exhibiting a better-defined physical structure, and methods of their fabrication, eliminating many of the sources of cross-correlation errors. More specifically, in the parent application, highly regular test structures are formed by selective etching of a monocrystalline precursor. For example, the sidewalls of conductors formed of monocrystalline materials on an insulative substrate can be fabricated to coincide with a single crystal lattice plane. The parent application teaches methods for using such highly-regular test structures for cross-calibration of optical instruments using electrical measurements.
As indicated, it will be recognized that the same problems inherent in measuring linewidth, and specifically in correlating measurements of linewidth made using one measurement technique with comparable measurements made using another technique, exist equally in connection with measurement of other geometrical characteristics of the structure, such as measurement of the spacing between adjacent conductors on a substrate, or of the overlay of different components of a composite structure formed in a succession of patterning steps. Here again, precision test structures would enable better correlation of various types of measurement instruments, as needed to evaluate more demanding production technologies being developed.
The present invention addresses a further specific need of the art. Optical "coordinate-measuring" instruments are widely used during semiconductor manufacture for measuring the distances between features in a plane, e.g., between the opposed side walls of a conductive line, between two parallel lines, between the corners defining the extent of a plane, or other features formed on a semiconductor substrate, a photolithographic mask, or the like. Such measurements are normally made by aligning fiducial marks (e.g. crosshairs) on a reticle of the instrument with the features the spacing of which is to be measured. Typically, a fiducial mark may be aligned with a centerline of a first feature and the object physically moved with respect to the instrument until the mark is similarly aligned with a second feature. Equivalently, pairs of spaced fiducial marks on the reticle may be simultaneously aligned with features on the object to be inspected, and the spacing of the features determined as a function of the magnification of the instrument.
Such measurement processes are only accurate if the magnification and the components for measuring motion of the optical instrument are satisfactorily calibrated, and if the fiducial marks on the reticle are reliably aligned with the features the spacing of which is to be measured. Generally similar instruments are used for placement of tools with respect to work in process, i.e., alignment of successive masks used in semiconductor production, and suffer from similar sources of inaccuracy.
Such instruments are commonly calibrated in two dimensions by alignment of fiducial marks on the reticle with spaced pairs of features on a reference object. Typically, this will be done by moving the reference object separately in two dimensions using x and y lead screws or similar mechanisms. At present there are no certified reference standards available that are suitable for conveniently calibrating such optical instruments in two dimensions.
More specifically, in order that the optical instrument can be used to measure dimensions of planar objects, such as photolithographic masks, semiconductor wafers or the like, in two dimensions, the instrument must be calibrated in two dimensions. However, the calibration objects now available for such use are not certified by reference to a suitable international standard.
More specifically, to properly calibrate such an instrument would require a reference grid defining a number of points in a plane, the distance between the points in two dimensions having been certified by measurement using an international standard measuring apparatus. Certified one-dimensional length scales are available, but are not satisfactory for calibrating an optical instrument in two dimensions, as is desired.
As noted above, the parent application (as well as various prior art disclosures) recognize that extremely precise structures--structures much more precise than the tools used in their fabrication--can be made by preferential etching of monocrystalline materials, and that this fact can be usefully employed in fabrication of test structures and the like. More specifically, it is known that structures of atomically-accurate planarity can be preferentially etched in monocrystalline silicon, as commonly employed as the starting material for semiconductor components. In the parent, this fact is used to form, for example, extremely precise elongated structures on a transparent substrate. The width of such structures can be measured by both electrical and optical techniques, enabling calibration of an optical instrument by reference to certifiable electrical measurements of the same structure.
As indicated, several prior art references suggest employment of the fact that monocrystalline materials may be preferentially etched along defined crystal planes to fabricate test structures, calibration structures, and like reference tools of great accuracy. Leone et al in "Fabricating Shaped Grid and Aperture Holes", IBM Tech. Disc. Bull., Vol. 14, No. 2, pp. 417-418 (1971) suggest that preferential etching of such materials can be used to form essentially perfect square apertures, and that these could be used to form a highly accurate calibration grid.
Young et al U.S. Pat. No. 4,885,472 discusses difficulties encountered in calibrating particle beam lithographic equipment using relatively inaccurate metal calibration grids. The improvement suggested by Young is principally in replacing the metal grid with a grid of silicon coated with a conductive metal layer, e.g., gold. The Young grid appears to be fabricated by preferential etching of a monocrystalline silicon precursor, and in essence comprises a number of the square apertures of Leone et al disposed in a two-dimensional array. The members remaining after the apertures are formed by preferential etching may each be expected to be atomically accurate, that is, the "wall" of each side of each aperture in the monocrystalline material will consist of a single crystal plane. However, as the walls of the various apertures making up the grid are not formed in a continuous etching step, they cannot be assumed to be coplanar, i.e., to correspond to the same crystal planes.
Accordingly, while certified measurements could be made along orthogonal lines of the spacing of the walls of the individual apertures making up the Young grid, these measurements would be insufficient to certify the locations of the intersections of the members defining the apertures. Hence the Young grid cannot be employed as a certified structure having a number of points at defined locations in a plane, and cannot be used in the manner described above to calibrate an optical instrument in two dimensions with respect to a certified standard for length. Neither Leone et al nor Young discloses a method of certifying the calibration grids thus formed, i.e., neither reference explains how the grids are to be o to a known reference standard.
U.S. Pat. No. 5,043,586 to Guiffre discloses a method for making calibration tools, in this case, artifacts having a number of metallic grid lines embedded in and coplanar with a substrate of a different material. A reactive ion etching process is used to form recesses in the surface of the substrate, which are then filled with the metal of the lines. By making reference to the Leone et al disclosure discussed above, Guiffre suggests forming such lines in a monocrystalline silicon substrate, such that extremely accurate lines would be formed. However, Guiffre's structure is not transparent and hence would not be useful in calibrating transmissive optical instruments, as are widely used; instead, only instruments operating on a back-scattering principle can be calibrated thereby. Furthermore, Guiffre's grid would only be suitable for calibrating an instrument with respect to measurements made in a single dimension; the test grid would have to be rotated through 90.degree., and the process repeated, in order to calibrate a two-dimensional measuring device. Finally, Guiffre does not disclose a method of certifying the calibration grids thus formed, i.e., does not indicate any method of comparing the actual spacings of the lines to a certified reference measurement.
Toda U.S. Pat. No. 5,264,696 shows formation of a cantilever probe structure for scanning probe microscopy. The Toda probe is formed using a precursor material comprising a silicon dioxide layer formed within a monocrystalline silicon substrate. A similar precursor material is used in certain embodiments of the present invention.
Other references generally pertinent to the subject matter of this application include Hatsuzawa et al, "Critical Dimension Measurements by Electron and Optical Beams for the Establishment of Optical Standards", Proc. IEEE Conf. Microelectronic Test Structures, Vol. 5, 180-184 (1992), showing formation of a deeply-finned structure by preferential etching of a monocrystalline silicon member. This structure is then used for cross-correlation of optical and scanning electron microscope measurements. See also Sickafus et al U.S. Pat. No. 4,808,260 showing etching of monocrystalline materials to form accurate apertures, e.g., for fluid flow nozzles. U.S. Pat. No. 5,485,080 to Larrabee et al shows nested quadrilateral structures formed on substrates which resemble structures formed according to the present invention, but used for entirely different purposes.
The art does not teach a two-dimensional structure formed on a transparent substrate having atomically-precise planar surfaces, which could be certified using a one-dimensional reference measuring instrument, and which would be useful in calibrating optical instrumentation in two dimensions.